Triboluminescence dominated by crystallographic orientation

Triboluminescence (TL) is an optical phenomenon that has a long and varied history with broad applications, such as damage detection, X-ray source, and mass health monitoring sensor. So far, the properties and mechanisms of TL remain not completely understood. The TL properties emitted during the sliding contact between Al2O3 and SiO2 surfaces were studied along different crystallographic orientations. In this study, the TL intensity of Al2O3 was significantly enhanced as Al2O3 surface was along a particular crystallographic orientation, which is an unconventional phenomenon. TL enhancement of Al2O3 was not affected by air atmosphere and atomic stocking mode of Al2O3. The enhancement mechanism of Al2O3 may be influenced by the surface state of Al2O3. This work provides a new method to control the intensity of TL and novel ideas to elucidate the TL mechanism.

Triboluminescence (TL) is a luminescence phenomenon by solid materials when they are stressed or fractured 1,2 . TL is also regarded as fracture, piezoelectric, and mechanical luminescence. Since the 20th century, this phenomenon has gained increasing attention because of its broad application 3 . TL has been successfully used in mass health monitoring sensor 4 , X-ray source, and damage detector 5 , among others. The triboluminescent properties of crystal materials are mostly investigated because many common crystal materials exhibit TL performance 6 .
TL properties of crystal are often affected by external factors, such as gas atmosphere, gas pressure, temperature, and speed. The photon emission intensity of Al 2 O 3 , ZrO 2 , and Si 3 N 4 with a diamond stylus decreases with increasing number of carbon atoms in the hydrocarbon molecules 7 , whereas the emission intensity enhances to a maximum value at a particular n-butane gas pressure 8 . The TL intensities of NaCl and LiF doped with Br, Sr, Ca, and Pb decrease with temperature and disappear completely at 105 ± 5 and 180 ± 10 °C, respectilvely 9 . Hollerman et al. 10 reported that the TL emission of Zn:Mn appears to be a function of speed for collision up to 6 Km/s. The doped impurities can change the TL properties of crystal materials; the TL emission intensity of ZnS:Mn is extremely strong, but TL properties are absent in ZnS 4,9,11 .
The physical properties of crystal structure also greatly influence TL. The discussion about TL properties of crystal with different space groups indicated that a non-centrosymmetric crystal structure is necessary but not sufficient for TL in crystal materials 12,13 . Hird 14 reported that the intensity of TL emission during diamond polishing in 'hard' direction is greater than 'soft' direction. The crystallographic orientation of Al 2 O 3 influences the atomic and electronic structures of alumina surfaces 15 . Brewer et al. 16 investigated the fluorescence band at 3.0 eV, which was produced by photoexcitation in high-purity Al 2 O 3 crystal. The results showed that the emitted light is plane polarized with the maximum intensity that occurs when the electric vector is perpendicular to the c axis of the crystal (E⊥ c) and with the minimum intensity that occurs when the electric vector is parallel to the c axis (E ∥ c) 16  They found that the stoichiometric surfaces of the C plane have the lowest surface energy, followed by the stoichiometric surfaces of the R plane and then the A plane. In other studies, Cs-corrected high-resolution electron microscopy that combines first-principle calculations and image simulations was used to observe and investigate the quantitative and qualitative structures of (1120) and (0001) surfaces 18 . Al 2 O 3 possesses good TL properties, but is not clearly investigated. SiO 2 is a common crystal material, and the investigation of SiO 2 of TL properties is very less.
The TL properties of crystal materials in different crystallographic orientations are rarely reported. We explored the TL properties of Al 2 O 3 along different crystallographic orientations by measuring TL emission during sliding with SiO 2 . We discovered an unconventional phenomenon that the TL intensity was enhanced several tens of times as Al 2 O 3 plane was in a particular crystallographic orientation. This work may provide a novel method to control the intensity of TL.

Discussion
The Al 2 O 3 crystal is a hexagonal crystal, and the side views of atom arrangement of Al 2 O 3 (0001), (1120), (1010), and (1102) surfaces are shown in Fig. 6(a-d), respectively. In the hexagonal unit cell, the atoms are stacked along the [0001] direction in a sequence of an oxygen layer and Al double layers: -AlAlO 3 -AlAlO 3 -R ( Fig. 6(a)    The results of rough measurements of surface charge of Al 2 O 3 and SiO 2 by using faraday cup 19 showed that Al 2 O 3 surface was negatively charged and SiO 2 was positively charged. Tribocharging mechanism is that electrons transferred from a surface with a low work function to a mating surface with a high work function 20 . In surface state theory, charge is exchanged between surface states in proportion to the difference between the effective or surface work functions of the two materials 21 . Surface potential difference is the fermi level difference between  original surfaces as well as the work function difference 22 . In equation (1), V C is the surface potential difference, φ 1 , φ 2 are the work functions of two surfaces respectively 23 .
SiO 2 surface has a lower work function than Al 2 O 3 surface 24,25 . Thus, electrons are transferred from SiO 2 to Al 2 O 3 , resulting in the former being positive and the latter being negative, then electric field between SiO 2 and Al 2 O 3 contacting surfaces is formed 26 . Electrons of ambient air molecules in electric field will be excited from ground level to the exited levels, then fall down to the lower or ground level, photons are emitted 26 . The sharp peaks of spectra are caused by the electrical breakdown of ambient gas 27 . The spectra peaks of photons in the region 300-450 nm are assigned to C 3 π → B 3 π electron transitions of N 2 28 . Other sharp peaks of spectra are mainly due to the B 3 π → A 3 Σ electron transitions of N 2 and the b 1 Σ + g → X 3 Σ − g electron transition in O 2 27 . The pressure of vacuum chamber is between 1 to 10 Pa during the sliding experiment. Air molecules are much smaller under this condition. Thus, Photons emitted during sliding between SiO 2 and Al 2 O 3 in vacuum is very few, the spectra of Al 2 O 3 in vacuum have no peaks.
As shown in Fig. 6(

Methods
Materials. SiO 2 and Al 2 O 3 crystals with trigonal and hexagonal crystal structures, respectively, were used in the sliding experiment. The two types of crystal planes of SiO 2 were (110) surface by X cut and (003) surface by Z cut. The results of single-crystal X-ray diffractometer of SiO 2 surfaces are shown in Fig. 7(a). Four surface planes of Al 2 O 3 crystal, including C plane (0001), A plane (1120), M plane (1010), and R plane (1102) 33 , were used as shown in Fig. 7(b). (1120) and (1010) planes are parallel to axis, and (0001) plane is perpendicular to axis, (1102) plane is crossed with C axis. The dielectric constant of sapphire at 298 K in 10 3 -10 9 Hz interval is ∥C = 11.5, ⊥ C = 9.3 34  Experimental setup. The schematic of the experiment setup used to observe the images and spectra of photons during sliding between SiO 2 and Al 2 O 3 is shown in Fig. 8. Optical fiber was used to gather light and then transmitted the light to a spectrograph (SP2500; Princeton Instruments, America). The images and spectra of photons were obtained with the spectrograph and CCD. The spectra of photons ranged from 300 nm to 900 nm, and the image of photons reflected the overall intensity of light. Experimental condition. SiO 2 was adhered to the rotating platform along with the motor, and the Al 2 O 3 wafer was fixed on a holder under a normal force of 10 N, as shown in Fig. 8. The bottom surface of Al 2 O 3 wafer was sliding over the top surface of SiO 2 wafer. The integration time (T) of CCD camera was 10 min, and the relative shear velocity (V) between Al 2 O 3 and SiO 2 wafer was 33 mm/s. The sliding experiment was performed in ambient air and vacuum. The vacuum pressure was between 1 and 10 Pa, and air humidity was nearly 10%. The red line in Fig. 8 is a removable wire line. The wire line connected the holder and the platform to reduce the influence of the external electrical potential difference. Each test was run three times, and mean was obtained to remove any discrepancies. The mean intensity of photon images is calculated by summing values of bright zone then dividing numbers of pixel points.